This invention relates to a composite material consisting of a non-metallic portion and a metallic portion. With the discovery of ceramic superconducting material and particularly with the discovery of ceramic superconducting materials having critical temperatures (T.sub.c) higher than the temperature of liquid nitrogen, a flurry of activity has occurred in the scientific community relating to these ceramic materials as well as newly discovered materials and the potential uses for them. Even with elevated T.sub.c s, at the present time the state of the superconductor art is such that superconducting materials must be cooled to a significant extent before the onset of superconducting properties, that is the point at which the electrical resistance becomes zero and there is complete diamagnetism. Taking into account the requirement of significant cooling, there will be difficulties in adhering superconducting ceramics to metal substrates due to the differences in the coefficients of expansion of the variant materials as well as thermal shocks created during repetitive thermal cycling between superconducting temperatures and ambient or elevated temperatures. Another significant requirement is that the superconductors be stabilized to provide an alternate current path if the superconductor goes normal. For example, if a portion of the superconductor, due to heat or other change, becomes a semiconductor, a large resistance is interposed in the path of the current carried by the superconductor. To avoid the possibility of explosion or other catastrophic event if the current carried by the superconductor is suddenly disrupted, the superconductor is provided an alternate current path by bonding a good electrical conductor to the superconductor. Hence, the superconductor is "stabilized". It is a further requirement that the interfacial electrical resistivity between the superconductor and the stabilizer be extremely low such that less than 10 watts/cm.sup.2 are generated during current shunting.
Various superconductor systems have been investigated including the 1-2-3 system and the 2-2-1-2 system, these respectively being the YBa.sub.2 Cu.sub.3 O.sub.x system and the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.x systems. However, the invention is applicable to any high temperature ceramic superconductor and includes the currently available thallium systems. Both of these superconductors are ceramics and the bonding of ceramics to metals has heretofore been difficult, if not impossible. There has been prior work involving the use of Li-Cu alloys to bond superconducting ceramics to metals such as copper or aluminum but the early work required relatively high temperatures to achieve bonding between the superconductor and the Li-Cu alloy. I have found that it is advantageous to avoid high temperatures when dealing with certain oxide superconductors since some oxide superconductors can be converted to a semiconductor at its interface with the bonding agent. This results in high electrical resistance at the interface. Another possible problem inherent in the use of lithium is that in contact with an oxide ceramic such as the 1-2-3 superconductor, the lithium tends to react with oxygen and convert the 1-2-3 superconductor to the YBa.sub.2 Cu.sub.3 O.sub.6 phase which is a semiconductor having a high electrical resistance.
Accordingly, it is a principal object of the invention to provide a composite material of a ceramic or glass portion and a metal portion wherein the composite is formed at temperatures less than about 400.degree. C. for instance 150.degree. C. or less using metals which do not react with the ceramic or glass.
Another object of the invention is to provide a method of bonding a metal to a non-metallic such as glass or ceramic at temperatures less than about 400.degree. C.
Another object of the invention is to provide a composite of a ceramic having a layer of silver or gold present at the surface thereof, a first metal bonded to the silver or gold, and a second metal bonded to the first metal.
Still another object of the invention is to provide a stabilized superconducting oxide ceramic having a layer of silver or gold at the surface thereof, a first metal bonded to the layer of silver or gold and a second metal bonded to the first metal, wherein the first metal bonds with the silver or gold atoms at a temperature less than about 400.degree. C. and the first and second metals bond at a temperature less than about 400.degree. C. and said first and second metals are good electrical conductors.
A final object of the invention is to provide a method of forming a composite including providing a ceramic having layer of silver and gold at the surface thereof, bonding a first metal to the layer of silver or gold at the ceramic surface at a temperature less than about 400.degree. C., bonding a second metal to the first metal at a temperature less than about 300.degree. C. to form a three layer composite wherein the first metal is selected from the class consisting of In, Ga, Sn, Bi, Zn, Cd, Pb, Tl and alloys thereof and wherein the second metal is selected from the class consisting of Al, Cu, Pb, Zn and alloys thereof.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.